1. Field of the Invention
This invention relates to a method of manufacturing a semiconductor single crystal by the Czochralski Method (hereinafter referred as the CZ method), and in particular to a method of manufacturing a semiconductor single crystal having a diameter of more than 200 mm and a uniform distribution of oxygen concentration along its axis.
2. Description of the Related Art
The substrate for a semiconductor device is mainly made of high-purity single crystal silicon that is conventionally produced by the CZ method. In the CZ method, polycrystalline silicon nuggets are fed into a quartz crucible of a single-crystal silicon pulling apparatus. Then the quartz crucible is heated by heaters disposed therearound to melt the polycrystalline silicon nuggets. Thereafter a seed crystal installed on a seed chuck is dipped into the melt. After that, the seed chuck and the quartz crucible are respectively driven to rotate in the same or reverse directions, and at the same time the seed chuck is pulled to grow a single-crystal silicon ingot of a predetermined diameter and length. FIG. 6 is a partial cross-section schematic view showing a apparatus for pulling silicon semiconductor single crystals.
As shown in FIG. 6, within the main chamber 1, a graphite crucible 3 is disposed upon the upper end of a rotary crucible shaft 2 which can be driven to extend upward or downward. Melt 5 (formed by making polycrystalline silicon melted) is charged in a quartz crucible 4 accommodated within the graphite crucible 3. A cylindrical heater 6 and a heat insulating barrel 7 made of adiabatic material are disposed around the graphite crucible 3. Furthermore, a supporting member 8 is installed on the upper end of the heat insulating barrel 7, and a gas stream guide 9 in a shape of a truncated reversed cone is installed on the supporting member 8. Inert gas such as Argon is guided from a pull chamber (not shown) connected to the upper end of the main chamber 1. The inert gas enters the interior of the gas stream guide 9 and goes down along the single crystal silicon 10. Then the inert gas passes through the gap between melt 5 and the lower end of the gas stream guide 9. By this arrangement, SiO.sub.x evaporated from melt 5 can be expelled outside of the main chamber 1.
Due to the fact that the surface of the quartz crucible 4 is in contact with melt 5, oxygen contained in the surface of the quartz crucible 4 will dissolve into melt 5. Most of the oxygen dissolved in melt 5 evaporates from the free surface of melt 5 and is expelled, together with inert gas, outside of the main chamber 1. However, part of the oxygen enters the single crystal silicon being pulled. Usually, the oxygen concentration in a single crystal silicon is high at the beginning of pulling and then decreases as the increases solidification ratio of the single crystal silicon.
It is well known that the concentration of oxygen contained in a single crystal silicon can be homogenized in the longitudinal direction by controlling the rotation speeds of the crucible and the single crystal silicon. For example, a method for manufacturing single crystal silicon was disclosed in JP-B- 3-21515 MEMC ELECTRONIC MATERIALS INC. USA! (the term "JP-B" as used herein means an published Japanese patent application). In the above method, the rotation speed of the single crystal is kept at a constant value greater than the maximum rotation speed of the crucible, and the maximum rotation speed of the crucible is controlled not to exceed the rotation speed of the single crystal in the event that the length of the single crystal is increasing. Thus, the distributions of oxygen concentration in the longitudinal and radial directions of the single crystal can be made uniform.
The above method is only suitable for the case where single crystals of a diameter less than 100 mm and a single crystal pulling apparatus without a gas stream guide are used. However, for single crystals with larger diameters than 200 mm, it is difficult to homogenize the distribution of oxygen concentration therein by only controlling the rotation speeds of the crucible and the single crystal. The reasons are: (1) Even if the rotation speed of the single crystal is kept constant, the moving speed in the peripheral direction at the boundary of liquid and solid will increase in proportion to the increase of the single-crystal diameter. There is a limit to the above moving speed in the peripheral direction that can steadily grow single crystals. In the case of a single crystal with a diameter of more than 200 mm, its rotation speed is limited to about 50% of that disclosed in JP-B 3-21515. (2) The peripheral speed of the quartz crucible also increases due to the enlargement of its diameter, and waves raised by the error in circularity of the quartz crucible or the eccentricity induced by dislocation of the quartz crucible during installation, on the free surface of melt become intense. Accordingly, it is difficult to obtain the steady growing of single crystals, and still the rotation speed is limited to about 50% of that disclosed in JP-B 3-21515. (3) If the rotation speed of the crucible is less than 5 rpm, natural convection in the melt, that is, the upward and downward convections become dominant, and horizontal convections are hindered. Therefore, oxygen concentration in single crystals can not be controlled, and the range in which the oxygen concentration can be controlled becomes very narrow. Accordingly, it is difficult to maintain uniform oxygen concentration along the longitudinal axis of a single crystal. (4) Depending on the design of the hot zones (melt portion), if the rotation speed of a single crystal is occasionally raised too high, the outer peripheral surface of the crystal will become uneven and no longer cylindrical. In the case of pulling a single crystal with a diameter of more than 200 mm, the maximum rotation speed of the crystal is about 20 rpm. Thus, even if it is intended to keep the oxygen concentration along the radial axis uniform, the rotation speed of the crucible can not be raised to half of that value of the crystal. Accordingly, the oxygen concentration in the single crystal can not be raised, and it is difficult to maintain a uniform oxygen concentration in the longitudinal direction.